Such a method is employed especially in machine tools and production machines such as multi-spindle automatic turning machines and rotary transfer machines, in which electric motors (preferably polyphase machines) are arranged on a rotationally moving machine part. The drive regulation and also the power electronics (line-commutated converter, self-commutated converter or unregulated DC converter and the AC power converter or converters) for driving the motors are generally arranged in a switching cabinet. The power lines and if necessary encoder lines (encoders in the motor for position and speed detection) are routed out of the cabinet to the motors. However the lines very quickly wear out as a result of the permanent bending and torsion stresses, with replacement of the lines being rendered very expensive by the mostly complex machine construction. In addition when using lines for energy transmission (i.e. especially between a fixed and rotationally-moving machine part) no “infinite rotation” of the moving machine part for further reduction of workpiece processing times is possible.
The function “infinite rotation” is currently becoming ever more attractive and there are already machines with this function. In such cases either the direct current and alternating current converter or only the alternating current converter are arranged on the moving machine part. The energy is accordingly transmitted as a two-phase DC voltage or three-phase AC voltage (mains voltage). The energy is currently preferably transmitted via slip rings, since inductive transmission means a significant additional outlay in electronics (as regards costs, installation volume and heat removal) including transformers (rotary transformers). A further variant is known from WO 2005/062432 A1 in which, instead of the conventional copper slip ring contacts, liquid metal alloys are used. With both variants the primary expense and cost factor is the number of energy transmission elements.
In accordance with the electrical regulations the protective earth conductor of a protective earth conductor system (for driving the motors) is to be additionally connected to the moving machine part and to the electronics located on it. According to EN 60204-1 (13.8.2 protective earth conductor circuit) the transmissivity of the protective earth circuit via sliding contacts must be guaranteed by the use of suitable measures (e.g. duplication of the current pickups, transmissivity monitoring). Thus either a transmissivity monitoring is necessary or two energy transmission elements are required for the protective earth conductor. In such cases these energy transmission elements must be designed in accordance with the operating current or regulations. (The outlay required for an energy transmission element with 200 A current carrying capacity is appr. 2000.00.)